The present invention relates to a twist drill for metal machining coated with a first and a second layer system, wherein the first layer system has been deposited over substantially the whole active part of the twist drill whereas the second layer system has been deposited only onto the tip area of the twist drill. The present invention also relates to a method of manufacturing such a twist drill.
Twist drills are often coated with a hard material to increase tool life by enhancing the wear resistance. A high wear resistance is advantageous at the tip area of the twist drill where the cutting takes place whereas properties like low friction and smoothness are advantageous on other parts of the twist drill, for example, to enhance chip transportation. The chip transport is particularly important when the drilled depth is relatively large compared to the nominal diameter. In some cases, by coating the chip flutes, the chip transport can, compared to an uncoated drill, even be decreased. Hence, achieving optimum properties for different parts of a twist drill can be difficult and several attempts have been made to accomplish this.
Another problem, which especially applies to twist drills coated along the cylindrical land, is referred to as pitting or pitting wear and includes spalling of the wear resistant coating on the cylindrical land and typically in the area close to the drill corner.
Pitting often includes not only spalling of the deposited coating but also part of the substrate from the twist drill. When the drill is used for the first time it is not a problem as the main cutting edge and the rake face, which are the areas determining the life time of the drill, are not affected by the pitting. The pitting appears at a small distance from the corner and the main cutting edge. However, when the drill is reconditioned it is often reduced somewhat in length and hence the worn region comes close to the cutting region and therefore the pitting can be detrimental to the life time of a reconditioned drill. As cemented carbide twist drills are very expensive it is necessary that it can be cut, reground and recoated several times and hence pitting has to be minimized to enable this.
Recently, new types of hard coatings usually with a multilayered structure, particularly useful for the wear protection of the main cutting edge and the rake faces, have been developed. Unfortunately, such coatings are very strongly prone to such pitting formation.
EP 0 983 393 B describes aperiodic multilayered coatings which leads to an increased hardness and increased wear resistance of the cutting edge. An example of such coating is alternating TiN and (Ti,Al)N layers having an individual thickness in the nanometer range, 3-100 nm.
JP 7237046A describes a method of depositing a coating onto a drill where the coating becomes thicker in the tip area compared to the rest of the drill body. The difference in thickness is achieved by partly covering the drill with a loose fitting metal cylinder which causes a gradual decrease in the amount of deposited material further away from the drill tip.
U.S. Pat. No. 6,688,817 B2 describes a drill that has been pretreated by applying an abrasive material to merely the tip, i.e. by a microblasting operation to roughen the surface. Thereafter substantially the whole drill is coated with a hard material. The coated drill will after coating have a larger surface roughness on the drill tip compared to the rest of the drill.
DE 196 02 030A describes a drill where only the tip has been coated, leaving the rest of the drill uncoated. The coating can be TiN, (Ti,Al)N or Ti(C,N).